1. Field of the Invention
The present invention relates to a webbing retractor which, during a sudden deceleration of a vehicle, is capable of instantaneously stopping the rotation of a take-up shaft in a webbing drawing-out direction by the operation of a lock mechanism which results from a lock wheel and the take-up shaft undergoing relative rotation due either to the engagement, with the lock wheel, of a pawl operated by sensing the sudden deceleration, or the sudden drawing out of the webbing at the time of sudden deceleration.
2. Description of the Related Art
Among webbing retractors that are mounted on vehicles, there is a type (which is called WSIR) which is adapted to operate a lock mechanism by sensing a sudden drawing out of the webbing and then making it possible to instantaneously prevent the drawing out of the webbing.
With this type of webbing retractor, when the webbing is drawn out suddenly by the occupant's inertia during a sudden deceleration of the vehicle, a delay in rotation normally occurs in a lock wheel which follows the rotation of a take-up shaft. This causes the lock mechanism to operate, thereby keeping the occupant in a restricted state by instantaneously preventing the drawing out of the webbing.
The cancellation of the lock mechanism operation is effected by rotating the take-up shaft by a small amount in a direction in which the webbing is taken up.
At this juncture, when the occupant unfastens the webbing, the take-up shaft is urged in the webbing take-up direction by means of an urging means. When the occupant releases the webbing, there is a case where the webbing is taken up quickly. In this case, since the rotation of the take-up shaft stops suddenly at a take-up limit of the webbing, relative rotation occurs in the lock wheel with respect to the take-up shaft due to inertia. Therefore, the state becomes identical to that in which the webbing is drawn out suddenly.
As a result, the lock mechanism is actuated which prevents the take-up shaft from rotating in the webbing draw-out direction. To cancel this state, it is necessary to rotate the take-up shaft by a small amount in the webbing taking-up direction, as described above. However, since a major portion of the webbing has already been taken up, there are cases where the cancellation is cumbersome and takes time.
In order to overcome the aforementioned drawback, a webbing retractor has been developed as disclosed in Japanese Utility Model Laid-Open No. 95058/1987. The webbing retractor is arranged in such a manner that a displacing member is disposed on a rotating member rotating integrally with the take-up shaft. The lock wheel is prevented from rotating in the webbing take-up direction relative to the take-up shaft by causing the displacing member to be engaged with the lock wheel as the take-up shaft rotates in the webbing take-up direction. In addition, the displacing member is disengaged from the lock wheel as the take-up shaft rotates in the webbing draw-out direction, permitting the take-up shaft to rotate in the webbing draw-out direction relative to the lock wheel.
On the other hand, there is another type of webbing retractor (VSIR) which is adapted to prevent the lock wheel from rotating by means of a pawl actuated by sensing a sudden deceleration of the vehicle, whereby a delay in operation normally occurs in the lock wheel which rotates by following the rotation of the take-up shaft so as to instantaneously prevent the drawing out of the webbing.
There is a demand to incorporate the functions of such a webbing retractor (VSIR) into a webbing retractor (WSIR) which is capable of preventing the rotation of the lock wheel in the webbing take-up direction relative to the take-up shaft at a take-up limit of the webbing in the same way as Japanese Utility Model Laid-Open No. 95058/1987.
In this case, however, when the rotation of the take-up shaft is stopped suddenly at the take-up limit of the webbing, there is the possibility of the pawl becoming engaged with the lock wheel due to a shock occurring at that time. In addition, if the webbing is subsequently drawn out in that state, the webbing drawing-out force can be imparted to the pawl via the lock wheel, possibly causing damage to the pawl.